When it does hit the fan, there’s a lot of it

Mint ST - - SCIENCE -

Sci­en­tists use the math­e­mat­i­cal model of defe­ca­tion and a de­flated bal­loon to de­code the mys­tery be­hind cube-shaped fae­ces pro­duced by wom­bats

DILIP D’SOUZA

though? Be­sides help­ing in di­ag­nos­ing uri­nary prob­lems, Hu and col­leagues also think it might “in­spire the de­sign of scal­able hy­dro­dy­namic sys­tems based on those in na­ture.” Briefly, a hy­dro­dy­namic sys­tem is one in which flu­ids are in mo­tion and that mo­tion it­self trans­fers en­ergy; an au­to­matic trans­mis­sion in a car is a good ex­am­ple. Who’d have thought that the way an­i­mals uri­nate might just have les­sons for how we can shift gears?

But uri­na­tion done with, Hu et al moved on to defe­ca­tion. Again they vis­ited the At­lanta Zoo. Again they filmed an­i­mals— cats to ele­phants—in the act. Again they dug through Youtube for 19 videos of an­i­mals defe­cat­ing. And among other things in their 2017 pa­per (Hy­dro­dy­nam­ics of Defe­ca­tion”, Soft Mat­ter, 2017, b.gat­ech.edu/2suwty1), again they found that an­i­mals “defe­cate within a nearly con­stant du­ra­tion”—this time, be­tween 5 and 19 sec­onds.

Is it sur­pris­ing that defe­ca­tion is more swiftly ac­com­plished than uri­na­tion? I’ll leave that for you to spec­u­late about.

Of course fae­ces, be­ing solid, doesn’t move through the in­tes­tine in the same way as urine moves through the ure­thra. a cat’s mere 4 cm.

But there’s more to the pic­ture. On av­er­age, an an­i­mal ejects two pieces of fae­ces. The length of each is about as long as the rec­tum, and about five times the di­am­e­ter of the rec­tum, but the “fae­cal and rec­tal di­am­e­ters match”. What this sug­gests is that the fae­ces is shaped in the in­tes­tine and then pushed through the rec­tum by mus­cu­lar con­trac­tions, or rec­tal pres­sure. This is a dif­fer­ent process from, for ex­am­ple, squeez­ing out tooth­paste, which is only shaped once in the noz­zle. In con­trast, if fae­ces is to be shaped in the rec­tum, it would need much more force than an­i­mals ex­ert dur­ing defe­ca­tion.

With all this con­sid­ered, Hu and his col­leagues ar­rived at their defe­ca­tion model. It ex­presses defe­ca­tion time in terms of the di­men­sions of the rec­tum, the rec­tal pres­sure, and the vis­cos­ity of the mu­cus the fae­ces picks up from the in­tes­tine. I’ll spare you the equa­tion it­self.

But in par­tic­u­lar, there’s no place in it for the mass of the an­i­mal. This is why the re­search shows defe­ca­tion time “within a nearly con­stant du­ra­tion” re­gard­less of the size of the an­i­mal. (The ac­tual slight de­crease in time, men­tioned above, is ex­plained by the greater vis­cos­ity of a larger an­i­mal’s mu­cus).

There’s more of in­ter­est in their pa­per, in­clud­ing at least one pos­si­bly in­ad­ver­tent turn of phrase that I’ll re­turn to. For now, I want to point out that the work Hu et al have done in­ves­ti­gat­ing the move­ment of fae­ces through in­testines has led them to an­other fas­ci­nat­ing dis­cov­ery. This has to do with the wom­bat, which is one of those over­whelm­ingly cute Aus­tralian an­i­mals. Now in pur­su­ing their wom­bat stud­ies, Hu et al didn’t visit the zoo or leaf through Youtube videos. In­stead, they got their re­sults from ex­am­in­ing “vet­eri­nary eu­th­a­nized (wom­bat) in­di­vid­u­als fol­low­ing mo­tor ve­hi­cle col­li­sions in Tas­ma­nia, Aus­tralia.”

The sci­en­tists took the in­testines from these dead wom­bats, emp­tied them and then in­flated them “with a long bal­loon”. They found that the bal­loon was sub­ject to dif­fer­ent strains at dif­fer­ent points in the in­tes­tine. This showed that the wall of the in­tes­tine has “az­imuthally vary­ing elas­tic prop­er­ties”—that is, the wall varies in elas­tic­ity, stretch­ing more at some places than oth­ers. Which is all very well, but what does this mean for wom­bat fae­ces? Some­thing amaz­ing in­deed: as the fae­ces pro­gresses through this in­tes­tine of vary­ing elas­tic­ity, it is shaped into … wait for it … cubes. (“How do wom­bats make cubed poo?”, 71st An­nual Meet­ing of the APS Divi­sion of Fluid Dy­nam­ics, 18 Novem­ber, bit.ly/2rpnifn).

Cube-like pieces of fae­ces, at any rate, with def­i­nite edges and cor­ners. Think of that. There is no other an­i­mal in this world that pro­duces cu­bic fae­ces, and how the wom­bat does it has for­ever been a mys- ad­van­tage while stak­ing out your par­tic­u­lar piece of real es­tate. And why study how wom­bats do this? Well, we hu­mans only know how to pro­duce shapes like cubes via mould­ing and ex­tru­sion tech­niques. Learn­ing how the wom­bat’s in­tes­tine works might give us, as Hu’s pa­per says, “in­sight into new man­u­fac­tur­ing tech­niques (for such struc­tures) us­ing soft tis­sues.”

Fi­nally, two more fae­ces-re­lated sci­en­tific find­ings.

The first is from, yet again, Hu and his col­leagues. They have es­ti­mated the amount of an­i­mal and hu­man fae­ces pro­duced an­nu­ally on the planet: 3.9 tril­lion kg in 2014, in­creas­ing at 52 bil­lion kg/ year. Why do this cal­cu­la­tion? Be­cause “hu­man and an­i­mal fae­ces present per­sis­tent threats to global pub­lic health and also op­por­tu­ni­ties for re­cov­ery of re­sources.” (“Es­ti­ma­tion of global re­cov­er­able hu­man and an­i­mal fae­cal biomass”, Na­ture, 13 Novem­ber, go.na­ture.com/2uhiybe).

The sec­ond is from a team of English and Aus­tralian health­care pro­fes­sion­als who run a blog called “Don’t For­get the Bub­bles” (DFTB). They know that chil­dren swal­low small ob­jects alarm­ingly of­ten, caus­ing par­ents much dis­tress. So they got sev­eral re­searchers to swal­low a Lego piece each and found that, on av­er­age, it took 1.71 days for it to ap­pear in their fae­ces. Their re­search fea­tures scores for “Stool Hard­ness and Tran­sit” as well as for “Found and Re­trieved Time”, and I’ll leave you to fig­ure out those par­tic­u­lar acronyms (“Ev­ery­thing is awe­some: Don’t for­get the Lego”, Jour­nal of Pae­di­atrics and Child Health, 22 Novem­ber 2018, bit.ly/2g1uear).

All in the ser­vice of science. Which is why I also want to point out that in the Hy­dro­dy­nam­ics of Defe­ca­tion pa­per, the au­thors tell us about a par­tic­u­lar char­ac­ter­is­tic that “ex­plains why dog fae­ces feel slip­pery when stepped on.” And in their Lego pa­per, the DFTB folks re­port: “There was some ev­i­dence that fe­males may be more ac­com­plished at search­ing through their stools than males, but this could not be sta­tis­ti­cally val­i­dated.”

Think of ev­ery­thing all this could set in mo­tion.

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